1. Field Of The Invention
The invention refers to an arrangement for characterizing magnetic coating compositions consisting of a dispersion of magnetic particles, particularly .gamma.-Fe.sub.2 O.sub.3 pigment, in a liquid resin-solvent mixture.
Such coating compositions containing magnetic particles are widely used for making magnetic record carriers, in particular magnetic disks and magnetic tapes. After the application of the magnetic coating composition on the carrier, the majority of the solvent evaporates. During or after the application of the coating composition and the evaporation of the solvent, there takes place an orientation of the magnetic particles, the firing and polishing of the record carrier, if necessary a lubrication of the surface, and finally a test for the magnetic characteristics of the record carrier. The quality of the magnetic coating composition is of decisive importance for the quality of the magnetic record carrier.
The magnetic coating composition is generally made using methods of paint production. Apart from precisely observing the chemical composition, it is of great importance to implement a complicated mechanical processing. This, and the physical-chemical characteristics of the components of the coating composition, as well as their interaction, determine the internal structure of the coating composition, with the degree of dispersion and of agglomeration of the individual particles playing an important part.
For these characteristics which strongly influence the quality of the finished record carrier there do not yet exist any direct measuring means. The viscosity of the coating composition which very much depends on the internal structure influences the processing characteristics. Electrostatic and magnetostatic forces between the particles can cause aging which only becomes visible later in the finished record carrier.
The quality of the coating composition is at present tested in production in that a control record carrier is made which substantially corresponds to a finished record carrier, and that the latter is then tested for quality. Further use of the coating composition depends on this check. It is obvious that such a test is very time-consuming, expensive and imprecise since immediately before, and during the application of the coating composition on the carrier, no measure of the quality of the coating composition is available.
Decisive factors for the quality of the magnetic coating composition are the degree of dispersion or of agglomeration, respectively, the particle density, and the viscosity. Furthermore, the stability of the composition plays a part which strongly depends on time, i.e. what is the stability of the coating composition during a predetermined period, or in other words, does the coating composition age prematurely, and if so, to what extent?
German Pat. No. 17 98 223 teaches a method of continuously controlling the homogenizing of mixtures composed of two or more substances. For that purpose, a powdery magnetically oriented substance is added as indicator to the mixture to be homogenized. By means of a measuring unit in the device, the intensity of the magnetic field is traced. With this method, the macroscopic dispersion of the indicator in the mixture is observed in which the magnetic density is measured. However, this method does not give any information on the microscopic distribution and the state of the individual particles.
German Offenlegungsschrift No. 20 49 463 discloses a method for the dispersion analysis of suspensions. There, the degree of dispersion of the suspension is determined according to the maximum layer thickness of a suspension applied on a substrate by means of a doctor blade, the substrate being moved continuously in one direction under the doctor blade. The disadvantage of this method is that for control purposes suspension has to be continuously withdrawn, and that furthermore it is very complicated to make a desired very low wedge layer thickness of a few .mu.m.
German pat. No. 23 37 165 describes a method and a device for measuring the electrokinetic zeta-potential of a dispersion. From the dispersion to be examined, a sample stream is continuously removed and sent through a separator cell, where it is exposed to a magnetic field and divided into several part streams. These part streams are subsequently directed through individual separate measuring cells where the solid composition content of the individual part streams is determined continuously. A measuring according to this method determines the electrostatic charge of the pigments with respect to the resin binder system, and it characterizes the stability of the dispersion but not its dispersion degree.
British Pat. No. 2059051 discloses a device for measuring the aggregation of particles at a wall, or among each other, the particles being dispersed in a fluid or gas flow. The fluid or gasous multi-phase system is directed onto a transparent wall and illuminated. The light which is scattered, reflected or attenuated by absorption is directed onto a detector, and evaluated. The measuring of the aggregation of the particles among each other, or at the wall works only with highly dilute dispersions that are transparent. It does not apply to highly concentrated dispersions as magnetic coating compositions in general, because these are opaque.
2. Summary Of The Invention
An object of the present invention is providing for the measuring of the properties characterizing a magnetic particle dispersion which is furthermore suitable of being the basis for a continuous measuring and control of the magnetic properties of the coating composition.
In the present invention, this object is achieved in that the coating composition is exposed to an alternating magnetic field of variable frequency, that the signal induced by the alternating magnetic field is received in the coating composition, and consequently that the susceptibility thereof is measured as a factor of the variable frequency, and evaluated with respect to the degree of dispersion, to the particle density, and to the viscosity of the coating composition.
The advantages presented by the invention substantially consist in that via the quality of the coating composition direct data can be supplied with respect to the characteristics degree of dispersion, particle density and viscosity.
This arrangement can thus be used for continuously controlling the characteristics and quality of the coating composition during its production and processing.